1. Field of the Invention
The present invention relates to a field of technology such as a chromatic dispersion measurement device that measures chromatic dispersion of a light pulse. Specifically, the present invention relates to a chromatic dispersion measurement device that measures chromatic dispersion of a light pulse propagating through a light transmission line of an optical fiber network in a high-speed optical communication system whose transmission rate is tens of Gbit/s, and a chromatic dispersion measurement method using the same.
2. Description of the Related Art
In recent years, data communication has shifted to communication via optical fibers and accordingly, a data transmission rate has become much higher than a conventional data transmission rate. In the near future, performing communication at a transmission rate of tens of Gbit/s or more, which is higher than current transmission rates, using an ultrashort light pulse will be considered in such a high-speed optical communication system via the optical fiber.
There is a problem in which crosstalk or a transmission error is always generated when data communication is performed in a high-speed optical communication system.
However, as the data transmission rate increases, a width of an individual light pulse and an interval of successive light pulses become narrow and the crosstalk or the transmission error described above becomes a very important issue, as will be described below, when high-reliability data communication is performed.
A speed at which light propagates through a material depends on a refractive index of the material, and as the refractive index increases, the speed of the light decreases. In materials such as glass, semiconductors or optical crystals, the refractive index changes with a frequency of the light (a wavelength in the air), and thus the light speed depends on the wavelength. It is known that, due to the wavelength dependence of the refractive index, a waveform of the light pulse is distorted and a time width of the light pulse is spread while a light pulse propagates through the material. Further, in an optical waveguide whose representative example is an optical fiber, an effective refractive index of the optical waveguide is determined according to a shape and a dimension of each of a core and a clad, and the speed of the light depends on the wavelength. Accordingly, a structure of the optical waveguide causes spreading of a time width of the light pulse. As described above, a characteristic that the light speed depends on the wavelength of the light is hereinafter called chromatic dispersion or simply dispersion.
The waveform of the light pulse is distorted or the time width of the light pulse is widened while the light pulse propagates through the optical fiber due to the above chromatic dispersion as described above, but this is not a particularly great problem since both the width of the light pulse and an interval of successive light pulses are wide at a conventional transmission rate in comparison with the chromatic dispersion.
However, when the data transmission rate is as high as tens of Gbit/s or more, the chromatic dispersion becomes wider than the interval of successive light pulses and crosstalk or a transmission error is generated. For example, successive light pulses interfere with each other. Accordingly, high-reliability data communication cannot be realized at a higher speed by an attempt to simply increase the transmission rate with current technology.
In order to remove (or control) the chromatic dispersion in the high-speed optical communication system described above, first, it is necessary to measure chromatic dispersion of various optical components used in the high-speed communication system and recognize a chromatic dispersion characteristic of each member.
For example, there is a chromatic dispersion measurement device that uses a spectral shearing interferometer using a frequency shifter, which measures spectral phases of various components in order to obtain chromatic dispersion from a change in the spectral phase (e.g., see Japanese Unexamined Patent Application, First Publication, No. 2007-085981).
In the spectral shearing interferometer, the interferometer is configured using a space optical system since two orthogonal components are simultaneously measured by converting cos and sin components of a light pulse into horizontal and vertical polarization components, respectively, and performing polarization division, to uniquely measure the spectral phase.
In the spectral shearing interferometer, a light pulse is propagated by linear polarization in an optical fiber constituting a part of the interferometer.
In the spectral shearing interferometer, it is necessary to convert linear polarization into circular polarization in order to generate the two orthogonal components of the cos component and the sin component.
The circular polarization is formed by superposition of two orthogonal polarizations of horizontal polarization and vertical polarization that are orthogonal vertically and horizontally. There is a phase difference of 90° between the horizontal polarization and the vertical polarization.
Accordingly, the two orthogonal components, i.e., the cos component and the sin component, can be obtained by spatially dividing the circular polarization into the horizontal polarization and the vertical polarization using a polarization beam splitter.
As described above, it is necessary to obtain the two orthogonal components of the cos component and the sin component in a plurality of wavelength bands for the measurement of the chromatic dispersion.
On the other hand, in the optical fiber, only light having a specific wavelength according to an optical length of the optical fiber is propagated without being changed from circular polarization to elliptical polarization, and light having other wavelengths is propagated with a change from the circular polarization to the elliptical polarization. Accordingly, the two orthogonal components cannot be maintained in a stable state and cannot be obtained with high accuracy.
Accordingly, the space optical system is used on an optical path related to the division of the two orthogonal components so that the circular polarization is not changed into the elliptical polarization. Light having all corresponding wavelengths propagate in a stable circular polarization and the two orthogonal components, i.e., the cos component and the sin component, are generated with high accuracy.
However, although the chromatic dispersion measurement device of Japanese Unexamined Patent Application, First Publication, No. 2007-085981 can accurately obtain the two orthogonal components of the light pulse, the chromatic dispersion measurement device uses the space optical system and light loss is generated by light input and output between the optical fiber and the space optical system. The light loss degrades intensity of the light and reduces measurement sensitivity.
Further, since the chromatic dispersion measurement device of Japanese Unexamined Patent Application, First Publication, No. 2007-085981 uses the space optical system, a configuration of the device becomes complicated and cannot be miniaturized due to an arrangement of parts necessary for the space optical system.
Further, since the chromatic dispersion measurement device of Japanese Unexamined Patent Application, First Publication, No. 2007-085981 can obtain the two orthogonal components through selection of the polarization using the space optical system, it is difficult to extend an optical system to generate a plurality of phase components rather than the two orthogonal components, such as three phase components whose phase angles are 0, π and α in radians. Here, α indicates any phase angle between 0 and π, that is, greater than 0 and smaller than π.
A non-interference component (a non-interference optical component, which corresponds to a DC component) that is a background component that degrades the accuracy of the chromatic dispersion characteristic can be removed through a mathematical operation using the three phase components 0, π and α described above.
However, in the chromatic dispersion measurement device of Japanese Unexamined Patent Application, First Publication, No. 2007-085981, only the two orthogonal components of 0 and π can be generated and no phase component with a phase angle α can be generated.
Accordingly, in the chromatic dispersion measurement device of Japanese Unexamined Patent Application, First Publication, No. 2007-085981, in order to remove the non-interference component, it is necessary to optimize the interferometer so that light intensity distributions in respective branch paths of the interferometer are always 50:50. Accordingly, a stabilizing mechanism for always maintaining the light intensity distributions in the branch paths to be 50:50 is necessary, and the device becomes complicated and large. Thus, it is difficult to realize a small chromatic dispersion measurement device.